The present invention generally relates to medical devices. More specifically, the present invention relates to medical devices suitable for intravascular ionizing radiation therapy.
Intravascular ionizing radiation therapy is being used increasingly to treat vascular disease. For example, intravascular ionizing radiation therapy has been proposed as both a primary and a secondary therapy for treating vascular restrictions. Clinical studies have shown that ionizing radiation may be used to inhibit or prevent restenosis after percutaneous transluminal angioplasty (PTA). In coronary applications, such vascular restrictions may range in length from a few millimeters to several centimeters, depending on the extent and nature of the disease, in addition to the size and type of vessel affected.
Typically, physicians evaluate the size (length and diameter) and nature of the vascular restriction in order to determine the appropriate treatment length and the corresponding ionizing radiation device to be used for treatment. However, intravascular ionizing radiation devices commonly utilize a fixed-length radiation source, and the number of different sizes available is limited. Accordingly, in some instances, the physician is not be able to select the correct radiation source length. Such limitations may leave the physician with no alternative but to select an intravascular ionizing radiation device having an improper radiation source length.
Thus, when utilizing an ionizing radiation device having a fixed-length radiation source disposed on the distal end thereof, clinical circumstances often give rise to a mismatch between the length of the radiation source and the length of the treatment site. Specifically, the length of the radiation source may be too long or too short as compared to the length of the treatment site. If the radiation source is too long, healthy tissue disposed at either end of the treatment site will inevitably be exposed to ionizing radiation. Exposing healthy tissue to ionizing radiation is clearly an undesirable event.
If the radiation source is too short, it is necessary to reposition the fixed-length radiation source in the vessel. Unless the treatment site is an exact whole number multiple of the length of the radiation source and the radiation source is precisely repositioned, various areas of the treatment site will inevitably have more or less radiation exposure than other areas of the treatment site. Repositioning the radiation source may result in overlapping exposure or healthy tissue exposure. Even if the treatment site length is an exact whole number multiple of the length of the radiation source, repositioning of a relatively short radiation source may be inherently imprecise and may require an increase in dwell time, both of which are not desirable. As a result, it is common for various regions of the treatment site to be either underexposed or overexposed to ionizing radiation due to a mismatch in length between the treatment site and the radiation source.
An example of a clinical implication of selecting an ionizing radiation device having an improper radiation source length has been observed, and is commonly referred to as the xe2x80x9ccandy wrapperxe2x80x9d effect. The xe2x80x9ccandy wrapperxe2x80x9d effect occurs when the length of the radiation source does not entirely cover the desired treatment site, thus, potentially leaving opposite ends of the vascular restriction untreated. The untreated regions of the vascular restriction may tend to restenos or re-occlude over time. The xe2x80x9ccandy wrapperxe2x80x9d effect may be compounded when a beta radiation source is used, due to the shorter depth penetration of beta radiation. As such, the conventional practice is to expose the treatment site to ionizing radiation beyond the proximal and distal ends of the restriction to avoid the xe2x80x9ccandy wrapperxe2x80x9d effect. However, such practice may expose otherwise healthy vascular tissue to potentially harmful ionizing radiation.
To address the issue of mismatched treatment length and radiation source length, ionizing radiation devices that utilize a moveable shield have been proposed. For example, U.S. Pat. No. 5,213,561 to Weinstein et al. discloses a device for preventing restenosis after angioplasty, wherein the device includes a radioactive source and a moveable shield in the form of an longitudinally shiftable sleeve to selectively expose the radioactive source. Shielded devices such as this allow adjustments to be made during the procedure in order to change the length of the exposure. Unfortunately, however, such fixed-length shielded devices tend to be stiff and relatively large, making it difficult to position the shielded portion of the device in tortuous or small diameter vasculature.
The present invention overcomes these disadvantages by providing an intravascular medical device including an elongate shaft and a variable-length ionizing radiation source disposed on the distal end thereof. The present invention may comprise a source wire, a guide wire, a catheter or other suitable intravascular device with an adjustable length radiation source. In one embodiment, the adjustable source is implemented on a wire having a variable-length radioactive tip. In another embodiment, the adjustable source is implemented on a balloon catheter having a variable-length balloon inflated with a radioactive fluid. In yet another embodiment, the adjustable source is implemented on a catheter having means for advancing and retracting radioactive seeds of variable number and/or spacing.
The intravascular device of the present invention provides a variable-length radiation source to compensate for any mismatch in length that would otherwise occur between a fixed-length radiation source and the treatment site. Thus, the variable-length design of the present invention improves the match between the radiation source length and the treatment site length. In addition, the variable-length design of the present invention reduces the inventory that must be kept on hand to treat different patients with different treatment site lengths, thereby reducing the otherwise significant cost of storing radioactive material. As compared to fixed-length devices of the prior art that require a shield to vary radiation exposure, the variable-length design of the present invention provides a more flexible and lower profile device by eliminating the need for a shield.
As implemented on a wire having a variable-length radioactive tip, the tip may comprise a coil, a braid, an elastomeric tube, or other suitable structure capable of longitudinal expansion and contraction. The radiation source may be formed of radioactive material, or the radiation source may be formed of an inert base material with a radioactive coating, dispersion, impregnation, composite, etc., thereon or therein. To affect variations in length of the source, the elongate shaft may include an outer tube and a movable core wire disposed therein. In this embodiment, the proximal end of the radiation source is connected to the distal end of the outer tube, and the distal end of the radiation source is connected to the distal end of the core wire. With this arrangement, longitudinal displacement of the core wire relative to the outer tube causes corresponding longitudinal displacement of the distal end of the radiation source, which in turn causes longitudinal expansion or contraction of the radiation source. Thus, both the length of the ionizing radiation source and the radioactivity per unit length may be varied.
As implemented on a balloon catheter having a variable-length balloon inflated with a radioactive fluid, the length of the balloon may be varied by a number of different means. For example, the length of the balloon may be varied by selectively retracting a constraining sleeve disposed about the balloon. Alternatively, the length of the balloon may be varied by utilizing an inverting balloon and selectively extending one end of the balloon. Any desired radioactive fluid and/or suspension may be used in conjunction with the variable-length balloon.
As implemented on a catheter having means for advancing and retracting radioactive seeds, either the number, spacing or both may be varied to effectively control the length of the radiation source. The seeds or pellets may be advanced and retracted by pneumatic means or by mechanical means.